or... func_illusionary. I'm not entirely sure but i believe these don't count towards the entity limit as they are static, not dynamic solid entities like a func_brush. There's no need to use a powerful (to a lack of better words) entity like a func_brush for such a simple task.

This... but keep in mind the brush wont have shadows. So if you want shadows there you need to make a wall the same size using the dev texture "block light". Then place the second a wall exactly where your false wall is.

Uh, Valve just seals off areas with skybox brushes or nodraw between displacements or carefully built func_detail pieces. Take a look at the sdk versions of Goldrush and Dustbowl. There's no occluding in the sky.

Not being able to see into other areas is the power of areaportals, which forcibly seperate areas of the map into individual segments (hence why they can cause your map to leak).

As yyler already pointed out skybox brushes seperate one area from another, though it's areaportals or cleverly placed hints that make sure you don't get odds and ends of brush faces being rendered around corners, behind skybox brushes.

I don't mean to be rude, but that is the idea behind most optimization. The fact that you thought occluders did this automagically for you was a bad sign, but the fact that you don't even know what optimizing does is frankly frightening.

A good start is to read this. Then go into the official maps and take a look at what they are doing. Use the SDK versions so that all the tool brushes remain intact.

A player standing in a room separated from the rest of the map should only be rendering what is in that room regardless unless there is some way to see from one to the other. If the map is sealed and the room is sealed there shouldn't even be an issue or cause for extraneous optimization.